In recent years, the flow rate of steam passing through the final stage of a steam turbine tends to increase as the provision of high output and high efficiency to the steam turbine progresses. To effectively expand steam as a working fluid, it is necessary that moving blades in a low pressure portion of the steam turbine are formed of long blades and an annular area is increased. But, when the moving blades are made long, a centrifugal stress increases and a natural vibration frequency decreases.
The centrifugal stress can be suppressed from increasing by, for example, an optimum distribution of the cross-sectional area of blades or provision of high strength to and weight reduction of the blade material. For example, the structure of the moving blade is devised in various ways for vibration characteristics, such that various characteristic values of the moving blades or moving blade group, which appear when the moving blades are made long, are detuned sufficiently relative to an operation frequency.
When the long blades are provided as independent blades, the detuning becomes difficult because characteristic values lie in various modes and frequencies. In response to the above, it is often that the moving blades of the entire annular circumference are determined as one group by forming a protruded portion on the moving blade tip portion to contact with the adjacent moving blade or using a connection part at the moving blade tip portion. In addition, there is a disclosed technology that the vibration characteristics are improved by disposing the same structure as that of the tip portion at an intermediate portion of the span from the blade root portion to the tip portion of the moving blade.
Especially, in a case where the connection structure is disposed at the span intermediate portion of the moving blade, the shape of the turbine moving blade cascade which is originally designed to suppress an aerodynamic loss as much as possible is deformed considerably or a resistance element is disposed in the flow passage between the moving blades. Therefore, it is obvious that the above situation becomes a factor of degrading the stage performance of the steam turbine. And, the suppression of the performance degradation is an issue to prove a highly efficient steam turbine.
Meanwhile, there is a disclosed technology that in a fluid machine using titanium having high strength, namely so-called specific strength, against the specific gravity of the material, as a material for the moving blade, a stress and a fluid resistance are reduced by having a pin which is small in mass and three-dimensional size as an intermediate connecting member. There is also a disclosed technology that an aerodynamic loss is reduced by having an airfoil shape for the intermediate connecting member of the fan moving blades. In addition, there is a disclosed technology that an aerodynamic loss is reduced by having a streamline-shape for the intermediate connecting member of the moving blades of the steam turbine.
FIG. 21A is a plan view showing a pressure side of a moving blade 300 of a conventional steam turbine. FIG. 21B is a plan view of a turbine moving blade cascade configured of the moving blades 300 shown in FIG. 21A seen from a radial outside. FIG. 21C is a view showing a V1-V1 cross section of FIG. 21B. The conventional turbine moving blade cascade shown here has the intermediate connecting member in a streamline-shape to reduce an aerodynamic loss.
FIG. 22A is a view illustrating a flow around a cylindrical intermediate connecting member 310 of the conventional turbine moving blade cascade provided with the intermediate connecting member 310. FIG. 22B is a view illustrating loss regions at a V2-V2 cross section of FIG. 22A. FIG. 23A is a view illustrating a flow around a streamline-shaped intermediate connecting member 301 at the conventional turbine moving blade cascade provided with the intermediate connecting member 301. FIG. 23B is a view illustrating loss regions at a V3-V3 cross section of FIG. 23A. FIG. 22B and FIG. 23B show the loss regions when the flows are observed from downstream sides at the individual cross sections. And, each two linear lines extended in a vertical direction shown in FIG. 22B and FIG. 23B indicate a trailing edge 300a of the moving blade.
The moving blade 300 shown in FIG. 21A is provided with the intermediate connecting member 301 on its suction and pressure sides as shown in FIG. 21B. The intermediate connecting member 301 has a streamline-shaped cross section as shown in FIG. 21C.
It is seen by comparing FIG. 22B and FIG. 23B that high-loss areas 320 expand largely due to twin vortices generated above and below the wake flow of the cylindrical intermediate connecting member 310. Meanwhile, the high-loss areas 320 decrease at the wake flow of the streamline-shaped intermediate connecting member 301 more than at the cylindrical intermediate connecting member 310, and low-loss areas 321 lie in a large area between the moving blades 300. It is seen from the above that the streamline-shaped intermediate connecting member 301 contributes to the reduction of an aerodynamic loss. But, the high-loss areas 320 have not disappeared completely, indicating that there is still scope for loss improvement.
Here, it is seen by observing the loss generating regions in detail relative to the moving blade 300 provided with the streamline-shaped intermediate connecting member 301 that they are deviated toward a suction side 300b of the moving blade 300 where the streamline-shaped intermediate connecting member 301 is connected. It is presumed to result from the generation of a low energy region when a boundary layer, which develops on the suction side 300b of the moving blade 300, crosses the leading edge portion of the intermediate connecting member 301. It is understood to be similar to a horseshoe vortex generated between the turbine moving blade cascades, and it is considered that the high-loss areas expand as the vortex develops in combination with the development of the boundary layer on the suction side surface having a continuous convex surface with respect to the flow. According to estimation such as numerical analysis, it is known that the stage efficiency might be lowered by several percent because of the above loss. For example, since an output sharing ratio to the entire steam turbine becomes 10% or more in the turbine stage provided with moving blades of long blade length in the steam turbine, the stage performance deterioration cannot be ignored.
As described above, when the intermediate connecting member is provided to improve, for example, the vibration characteristics of the moving blades which are long blades, it becomes a passage resistance against the steam flowing between the moving blades, and aerodynamic performance is lowered.
For example, when the intermediate connecting member is reduced in three-dimensional size in order to suppress the above, a risk of buckling distortion or breakage increases at the intermediate connecting member or the connection portion between the intermediate connecting member and the moving blade because a section modulus to an untwisting force of the blade is insufficient. And, in a case where the intermediate connecting member is configured into a streamline-shape, the high-loss area is not eliminated even if the streamline-shape is formed while member strength is secured.